[PDF] Improve PDF support in gm.
[PDF] Improve PDF support in gm.

Currently gm ends up writing the same PDF four times.  This fixes that problem, making PDF a first class citizen and providing a better framework for additional backends.  Read support for PDF is still missing though.

Review URL: http://codereview.appspot.com/4130045

git-svn-id: e8541e15acce502a64c929015570ad1648e548cd@759 2bbb7eff-a529-9590-31e7-b0007b416f81

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